The majority of human neoplasms is thought to be due to environmental factors. There is overwhelming evidence that skin cancer, one of the most commonly occurring human cancers, is usually due to exogenous insult. The cells that are most likely to be progenitor cells for this carcinoma are the basal cells, which are rapidly dividing and/or differentiating, and as such make an excellent target cell for carcinogenic agents. The purpose of this work is (1) to develop an in vitro system consisting of dividing, basal cells isolated from hamster epidermis, (2) to induce these cells to undergo neoplastic transformation in vitro following treatment with environmental carcinogens and/or promoters, (3) to determine whether the two-stage model of carcinogenesis can be extended to Syrian hamsters who are not sensitive to phorbol ester promotion of epidermal carcinogenesis.